PURPOSE: To improve the reliability of a compressor by, when a difference in pressure between a high pressure chamber and a low pressure chamber becomes larger or temperature in the high pressure chamber becomes higher during the operation of the compressor, checking the flow-in of exhaust gas fluid from the high pressure chamber to the low pressure chamber for preventing the lowering of performance, the abnormal pressure rise of the exhaust gas and the deterioration of oil.

CONSTITUTION: In a scroll compressor whose casing 1 is divided into a high pressure chamber 12 and a low pressure chamber 14, an oil returning passage 7 for collecting an oil gathering in the high pressure chamber 12 into the low pressure chamber 14 is provided between the high pressure chamber 12 and the low pressure chamber 14, and the said passage 7 is provided with a differential actuating valve 8 which opens the oil returning passage 7 by its opening action when a difference in pressure between the high pressure chamber 12 and the low pressure chamber 14 is less than a fixed value and closes the passage 7 by its closing action when the difference in pressure between them is more than the fixed value.